This invention relates to air cushion supported vehicles, and more particularly to marine vessels of such type which are sometimes referred to as surface effect ships ("SES"). Specifically, the invention relates to air cushion seals comprising one or more inflatable bags or "lobes", such as may be used at the stern of a "SES" to extend between the sidehulls or skegs of the ship as shown for example in U.S. Pat. Nos. 3,977,491; 4,090,459; 4,137,987; 4,303,034 and 4,333,413. Also, such seals may be employed at the bow of the ship as well as between air cushion space partitioning keels or skegs or the like for stability improvement purposes, such as shown for example at 18,20 in U.S. Pat. No. 4,090,459.
Such inflatable seals may comprise any number of such lobes arranged in superposed relation. In any case, the seal device is hingedly affixed at its forward end to the hull structure so as to bridge the air cushion space beneath the ship; the major body portion thereof being thereby suspended to provide an inclined hanging curtain riding at its bottom upon the water surface over which the ship is operating for maintaining the air cushion support under the vessel.
The recent development and advent of relatively high speed operating vessels of this type has presented a new problem to the art in that such seal structures are now called upon to have much more rapid responses to the more rapidly encountered variations in the contours of the water surface over which the vessel is travelling. Relatively non-compliant seal structures such as have been previously successful are in this case subjected to unacceptable "snap-back" and "slamming" loads on the seal structure and drag forces on the ship operation. Thus, to accommodate varying sea state conditions it is therefore required that the seal structure in a high speed surface effect ship be relatively lightweight and versatile as to its ability to adequately function in accordance with its primary purpose, while at the same time being structurally competent to withstand the impacts and dynamic loads imposed upon the structure under high speed operating and severe sea state conditions.
The present invention provides an improvement as to prior art approaches to this problem, whereby such prior art type relatively lightweight seals which are prone to undesirably vibrate and bounce on the surface of the water when operating at high speeds over rough water may be modified to perform as required. This tendency to "bounce" upon the water surface is due to the "throttling" effect which takes place between the seal structure and the water surface in accordance with the well known Bernoullis theorem. Furthermore, the bouncing oscillation tendencies of the lowermost lobe in a seal construction of this type versus the water surface induce the so-called "Coanda effect" which operates on the lowermost lobe of the seal. Thus, the lowermost lobe of the seal is prone to vibrate and bounce on the surface of the water. Under certain conditions, this motion induces sympathetic motions in the other elements of the seal to such an extent that the natural frequency of the seal is obtained. At this point, the various elements, or lobes of the seal are prone to vibrate and bounce on the surface of the water and to slap together and apart at the natural frequency of the seal. This operates to generate resonate vibrations such as are seriously detrimental to the seal constructions of the prior art, and are discomforting to the ship passengers and crew. This invention provides means to counter such tendencies of relatively lightweight seal structures to vibrate and bounce upon the surface of the water over which the ship is travelling, while still performing its primary air cushion sealing purpose.